1. Field of the Invention
This invention relates to an improved high-pressure process for effecting exothermic or endothermic gaseous reactions such that maximum gas temperatures are always in the core of the apparatus utilized for such process and minimum pressure drop conditions and significant economies can be achieved, resulting in extension of catalyst life and a marked decrease in the capital cost of such apparatus.
More specifically, this invention relates to an improved high-pressure process, capable of effecting either exothermic or endothermic gaseous reactions with the foregoing results which utilizes a single-walled pressure vessel or shell containing cross-flow, as, e.g., radial flow, heat transfer exchangers, a continuous particulate catalyst bed having at least two stages, and means for effecting "cross-over" material flows such that, for exothermic reactions, material flows are radially directed inwardly from "outside" to "inside", whereas, for endothermic reactions, material flows are radially directed outwardly from "inside" to "outside".
2. Description of the Prior Art
Heretofore, the art has been replete with high-pressure processes and catalytic reactors or converters for effecting the gaseous syntheses of such valuable materials as ammonia, methanol, hydrocyanic acid, hydrogen, methane, and styrene. Typically, such apparatus have had to be built to withstand the extreme pressures and temperatures associated with such syntheses, approximating wide limits varying, for example, from between 1200 to 10,000 p.s.i.g. Thus, in order to accommodate the commercial production rates required, e.g., catalytic converters capable of generating 1000 tons of ammonia per day, double-walled reactor vessels of enormous size have had to be employed as shown in U.S. Pat. No. 3,567,404. However, the costs and difficulties of manufacturing such converters have likewise been enormous. Moreover, equipment sizing problems have also been encountered, since in order to maintain space and linear velocity conditions at reasonable pressure drops, converters of prohibitively large diameters, in view of their high operating pressures, are required. Furthermore, it is well-known in the art that, for a given operating pressure and temperature, the larger the diameter of the vessel, the thicker its walls have to be. Since the materials of vessel construction are also influenced by the temperature as well as by the hydrogen partial pressure, the reason for use of conventional double-walled vessels in the part has been manifest.
Accordingly, the art has long been concerned with providing reactors or converters of increased production for high-pressure processes suitable for large-scale reactions, within the limits of acceptable design criteria and having flow patterns of reactants which lend themselves to increased production through increased length of the reactor or converter rather than through an increase in such reactor's or converter's diameter.
This too has posed problems in view of the fact that, in order to accommodate the increased production requirements for such processes, tall reactors or converters on the order of 40-50 feet high are required. Since within such reactors or converters one or more beds of catalytic contact material has to be vertically disposed, maintenance of optimum space and linear velocity conditions without prohibitive pressure drops has not been attainable, and various means have been sought to solve this problem.
One such solution, with respect to such process deficiencies, has been proposed in U.S. Pat. No. 3,567,404, which utilizes the conventional double-walled reactor, whereby the reactant gases are permitted to flow in a directional perpendicular to the longitudinal axis of the outer shell and the inner reaction zone, and across one or more catalyst beds in series, such that the gases flow from one bed to the next consecutive bed through a passageway therebetween, the direction of flow of the gases through said passageway being generally opposite to their direction of flow through the catalyst bed. The arrangement of flow in this manner greatly facilitates the manner in which the reaction is conducted and permits wide alteration of desirable variables. For example, by having reactant flow downward across one bed and upward through an adjacent bed, this flow pattern has the effect of shortening the converter by eliminating the passageways between the beds. However, such flow methods and patterns have been unsuccessful because they have been unable to satisfy the temperature requirements associated with optimal yields and maximum suppression of competing side reactions, notwithstanding the use of heat exchange means disposed to accommodate such flow methods and patterns. Moreover, these flow patterns are subject to increased flow resistance, thereby leading to increased pressure drops, and a considerably reduced circulation rate through the reactor for a given catalyst volume. The solution to this type of flow-type, process problem has been the adoption of radial flow means such as taught by (1) U.S. Pat. No. 3,372,988, which originated the idea of "means for passing a synthesis gas through the catalyst bodies successively in opposite radial directions"; and by (2) an improved version of radial flow in U.S. Pat. No. 3,472,631 whereby the reactant gases are made to flow through each successive catalyst bed layer more or less horizontally in the reverse direction to the preceding catalyst layer and around heat exchange tubes at turning points counter-current to the fresh reactant gases.
Additionally, the concept of circulating feed gas through tubes disposed in the catalyst bed for cooling purposes, prior to actual contact of the feed gas with the catalyst, has been shown in U.S. Pat. Nos. 2,853,371; 3,041,161; 3,050,377; and 3,212,862. The alternative approach to this mode of cooling has been through the use of quench cooling and quench-type converters as shown in U.S. Pat. Nos. 2,495,262; 2,632,692; 2,646,391; 3,366,461; 3,396,685; 3,433,600; 3,433,910; 3,458,289; 3,475,136; 3,475,137; 3,498,752; and 3,663,179. In the prior art quench-type converters, the quench fluid has generally been added to the main reactant stream between separate beds consisting of solid catalyst granules, spheres, or the like. The quench-cooled apparatus, however, have suffered from the disadvantages of high pressure drop and increased cost and complexity.
Heretofore, however, none of the prior art high-pressure processes, or catalytic apparatus utilized therein, for performing reactions in the gaseous phase have been effective for both exothermic and endothermic reactions; none have known that it was possible to use single-walled apparatus adapted to accommodate radial material flow patterns integrated with cross-flow heat exchangers. Moreover, none of the known apparatus have been successful at maintaining maximum gas temperatures in the core of the apparatus with minimum pressure drop conditions within the limits of acceptable design criteria and acceptable flow patterns of reactions. In particular, and most notable is the fact that the prior art has been unaware of the use of cross-flow, e.g., radial, heat exchangers (let alone of their use with single shell reactors), thereby to enable flow direction to be arranged to make gas expansion or contraction consistent with catalyst cross-section expansion or contraction. The present invention has been developed to fill this void, and it does so through means of processes which employ a new conceptually-based design of apparatus which enables the use of a single-walled reactor or reactor system having multiple reaction stages, whereby a radial flow of reactants is developed in accordance with a "cross-over" pattern such that material flows are directed from "outside" to "inside" for exothermic reactions and vice versa for endothermic reactions. For exothermic reactions, one form of the present process provides one-cross-flow heat transfer stage for each reaction stage. However, in another form of the present process concerned with endothermic reactions, the first heat transfer stage is external to the system (for example, it can be situated outside the reactor), and hence for such endothermic reactions, there are one fewer heat transfer stages than reactor stages.